Update to git:51dafca

Changes:
    core: introduce grace period
    dns: iterate over all configured nameservers
    doc: add missing documentation
    doc: add missing documentation
    mtr.8: Use two whitespace after all full stops
    mtr.8: Improve help on -i
    Merge Richard Hartmann's tweaks to the man page
    Fix remaining single-space after stop in manpage
    Direct contributors to github in man page